Deleting array elements in python

Question

All, I want to delete specific array elements of one array from the other. Here is an example. The arrays are a long list of words though.

A = ['at','in','the']
B = ['verification','at','done','on','theresa']

I would like to delete words that appear in A from B.

B = ['verification','done','theresa']

Here is what I tried so far

for word in A:
    for word in B:
        B = B.replace(word,"")

I am getting an error:

AttributeError: 'list' object has no attribute 'replace'

What should I use to get it?

Foo Bah · Accepted Answer · 2011-09-18 03:14:41Z

3

Using a list comprehension to get the full answer:

[x for x in B if x not in A]

However, you may want to know more about replace, so ...

python list has no replace method. If you just want to remove an element from a list, set the relevant slice to be an empty list. For example:

>>> print B
['verification', 'at', 'done', 'on', 'theresa']
>>> x=B.index('at')
>>> B[x:x+1] = []
>>> print B
['verification', 'done', 'on', 'theresa']

Note that trying to do the same thing with the value B[x] will not delete the element from the list.

answered Sep 18, 2011 at 3:14

Foo Bah

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3 Comments

Patrick Perini Over a year ago

Also, B.pop(x), or even better B.remove('at') would work here.

warvariuc Over a year ago

B[x:x+1] = [] -> del B[x:x+1]

Foo Bah Over a year ago

@Patrick one fringe benefit of presenting this array technique is that the same form can be used to insert multiple elements. For example, x=[1,2,3];x[1:2]=[5,6,7];x returns [1,5,6,7,3].

Marat · Accepted Answer · 2011-09-18 13:20:48Z

3

If you are ok to delete duplicates in B and don't care about order, you can stick up with sets:

>>> A = ['at','in','the']
>>> B = ['verification','at','done','on','theresa']
>>> list(set(B).difference(A))
['on', 'done', 'theresa', 'verification']

In this case you'll get a significant speedup as lookup in set is much faster than in list. Actually, in this case it's better A and B to be sets

edited Sep 18, 2011 at 13:20

answered Sep 18, 2011 at 4:35

Marat

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4 Comments

agf Over a year ago

"lookup in set is much faster than in dictionary" -- I'm not sure quite what you mean here, as I don't see dictionaries in any of the other answers, but lookup in a set is basically the same as key lookup in a dictionary.

steveha Over a year ago

I think he meant "lookup in set is much faster than in list". The question is about lists, everyone is talking about lists, and this way it is a correct statement.

Marat Over a year ago

you're right, it's all about lists. Lookup in lis is O(n), lookup in set/dictionary is O(1)

OneOfOne Over a year ago

However the result's order will be changed.

OneOfOne · Accepted Answer · 2011-09-18 03:44:31Z

1

You could also try removing the elements from B, ex :

A = ['at','in','the']
B = ['verification','at','done','on','theresa']
print B
for w in A:
    #since it can throw an exception if the word isn't in B
    try: B.remove(w)
    except: pass
print B

answered Sep 18, 2011 at 3:44

OneOfOne

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